The present invention relates generally to a method and an apparatus for welding a spherical metal to a matrix metal using electrical-resistance welding. More particularly, it relates to a method and an apparatus for welding a ball member to an end of a matrix cylindrical body to make a plunger which is adapted for use in a directional control valve for controlling fluid communication in the valve.
For aiding in understanding of the present invention and an application thereof, reference is made firstly to FIG. 1 which shows a typical solenoid-operated directional control valve 51 which has formed therein a first fluid passage 56, a second fluid passage 57 and a third fluid passage 59 and also contains therein a displaceable cylindrical plunger 52 of prior art having joined to its opposite ends a pair of spherical balls 55, 60. In normal position of the valve 51 when its solenoid 53 is in deenergized, or off, state, the plunger 52 is urged by the tension of a coil spring 54 to be positioned such that the ball 55 on the right end of the plunger closes the port of the first fluid passage 56 and the other ball 60 on the left end of the plunger keeps open the port of the second fluid passage 57, so that the third fluid passage 59 is in communication with the second passage through a plunger chamber 58 in which the plunger 52 is displaceably mounted, as shown in FIG. 1. When the solenoid 53 is energized, on the other hand, the plunger 52 is shifted leftward by the electromagnetic force created by the then energized solenoid while overcoming the spring tension until its ball 60 on the left end closes the port of the second fluid passage 57 to shut off its communication with the plunger chamber 58, and the first fluid passage 56 is opened to the plunger chamber 58 so that a fluid communication is then made between the first and third passages 56, 59.
Referring specifically to the plunger 52 of the prior art, it has formed at each end thereof an inward recess 61 having respective spherical surfaces conforming to those of the balls 55, 60 for receiving therein part of the ball. As indicated by solid lines in FIG. 2, the recess 61 is formed with its center located on the axial center line L1 of the plunger 52, which center line L1 is in turn in alignment with the axial center lines L2, L3 of the first and second fluid passages 56, 57 in the valve, respectively, as shown in FIG. 1, so that the balls 55, 60 joined to the cylindrical plunger body close sealingly the ports of the fluid passages 56, 57.
According to a conventional method to fix the balls, each ball is placed in its corresponding recess 61 and joined to the plunger body 52 by laser welding the adjoining area between the ball and the radially outer circular edge of the recess. The welding is started from any convenient location, e.g. a point as indicated by arrow A in FIG. 2, and performed along and throughout a circular line of the above adjoining area.
In the laser welding which is carried out in a continuous manner along the circular line and causes the metal to be fused or melted by application of welding heat, however, a portion welded earlier naturally solidifies earlier than the other portions which are welded later. In welding the balls to the plunger body, the difference in solidification time produces a tensile stress in the weld which may cause the ball to be displaced radially as indicated by a phantom circle in FIG. 2. Such displacement of the ball causes its center line to be offset from the center lines L1-L3 of the plunger body and hence from the first and second fluid passages 56, 57, with the result that the ball fails to seal tightly its corresponding fluid passage port.
Furthermore, application of the welding heat by the laser welding for fusion of the metal may cause deformation of the balls 55, 60 from their sphericalness, thereby affecting the sealing performance of the plunger in the directional control valve and hence the operation of the valve.
For achieving satisfactory fluid-tight sealing of the fluid passage ports in the valve, it is important that the plunger body 52 have as little error as possible in its axial dimension and also that the balls 55, 60 should be as true as possible in their sphericalness. For the latter purpose, steel balls similar to those employed in a ball bearing may be advantageously used for their high precision in sphericalness. However, these balls are usually made of a high carbon steel and, therefore, offer a disadvantage in that such ball when laser-welded and then cooled is susceptible to harmful cracks.